Many types of skeletal implants and skeletal joint prostheses are available as substitutes for natural skeletal components and joints. Such prostheses are used when the natural skeletal component has been damaged by disease or trauma. However, a problem exists in the case of juvenile patients or other patients where natural growth would otherwise be occurring. As the patient grows, the limb having the prosthesis implanted will not grow at the same rate as the opposing limb unless the prosthesis allows for growth along its length.
The U.S. Pat. 4,892,546 to Kotz et al. issued Jan. 9, 1990 discloses an adjustable prosthesis for a bone joint including a joint component, an elongated rod having a distal end and proximal end, a drive trunion connected to the rod through an angular gear, an elongated inner sleeve and outer sleeve, and means for preventing rotation of the inner sleeve within the outer sleeve. The elongated rod includes a threaded spindle and the inner sleeve includes a threaded spindle nut which cooperates with the threaded spindle to provide extension and retraction adjustment of the rod relative to the inner sleeve. The European patent application EP 86109228, assigned to Waldemar Link GMBH, discloses an endoprosthesis replacing bone middle sections including a tubular spacer with a collar at each end and a solid bone nail extending from one collar. A sliding nail fits in a bore of the tubular spacer and can be locked in any required position by a lock screw.
A joint implant is disclosed in the U.S. Pat. 4,502,160 to Moore et al, issued Mar. 5, 1985, and assigned to the assignee of the present invention. The Moore et al. patent discloses an implant including a stem for bone implantation including a threaded portion for adjusting axially with respect to an overlying sleeve which carries an articulating component of the joint. A pin in the sleeve and an elongated slot in the stem restrain relative rotation but allow axial relative movement as the stem is urged into or out of the sleeve by the rotation of a nut coacting with the threads on the stem while being axially restrained by the sleeve. The nut acts as a ring gear.
Although the Moore et al. patent discloses a very useful embodiment for the prosthetic joint implant, there are several problems that have been encountered with its use. First, the nut interconnecting the stem with the sleeve or barrel is held on the sleeve by two pins which engage an annular slot disposed about the outer surface of the sleeve. This connection fixes the nut axially relative to the sleeve while allowing rotation of the nut, the rotation extending or retracting the stem member. However, the two pins provide a limited two point contact area between the nut and the barrel about which the nut is rotating. There is a torquing force on the pins which results in friction, thereby increasing the torque necessary to rotate the nut. Further, the round pin ends on the flat surface of the groove on the sleeve causes further friction. A second problematic area results from the end of the barrel contacting an inner surface of the nut. Again, this creates a force or friction which must be overcome. Since these parts are generally made out of biocompatable titanium and titanium is susceptible to galling, such metal to metal contact and the resulting friction should be avoided. However, the juxtaposition of the inner surface of the nut and end of the barrel is necessary since there is only a two point contact between the means retaining the nut on the barrel and the barrel itself.
An additional problem is galling of the threads of the stem caused by the threaded engagement and forces with the threaded inner surface of the nut. These strains can result in increased wear of the unit.
The present invention provides an improvement over the implant disclosed in the Moore et al. patent by providing means for reducing and minimizing the relative friction between the nut member and the sleeve as well as increasing fatigue strength of threaded areas of the device thereby decreasing wear of the parts.